The chemical composition of myelin is well established and one of the principal protein constituents of the membrane is the myelin basic protein (MBP). Recent evidence suggests that the MBP is synthesized as a precursor which undergoes post-translational processing to its "mature" form. The purpose of this proposal is to study the in vivo and in vitro synthesis of the myelin basic proteins in the developing mouse brain and their assembly into myelin. The overall objective of the work is to define the molecular sequence of events leading from the site of synthesis of the myelin basic proteins and their sites of conversion from precursor to mature forms, through their incorporation into any precursor membranes, to their ultimate assembly into myelin. To accomplish this the development of an immunoabsorbent column is proposed to facilitate the isolation of MBP-related proteins from both whole brain and subcellular fractions. Using such a methodology, synthesis of the basic proteins will be studied in vivo and in several in vitro systems, including brain slices, homogenates, and purified ribosomal systems. These systems will be used to define the metabolic relationships which exist between the precursors and mature MBPs by a variety of approaches including kinetic studies on the uptake of labelled amino acids into individual proteins and pulse-chase and inhibitor studies to establish precursor-product relationships. Studies on the biogenesis of myelin using these proteins as probes will be accomplished by essentially the same kinetic approaches by following uptake of label into specific MBP related proteins in different subcellular components including the myelin subfractions. Precursor-product relationships will be established using inhibitors and pulse-chase experiments. These studies will be supplemented by others aimed at determining the subcellular location of the "processing" enzymes which convert the precursor MBPs into their mature forms. This work should provide important fundamental information, not only on the biogenesis of myelin but also on the assembly of proteins into membranes, in general.